1. Technical Field
The present invention relates in general to earth-boring bits and, in particular, to an improved bit having a combination of rolling-cutters and fixed cutters and cutting elements and a method of design and operation of such bits.
2. Description of the Related Art
The success of rotary drilling enabled the discovery of deep oil and gas reservoirs and production of enormous quantities of oil. The rotary rock bit was an important invention that made the success of rotary drilling possible. Only soft earthen formations could be penetrated commercially with the earlier drag bit and cable tool, but the two-cone rock bit, invented by Howard R. Hughes, U.S. Pat. No. 930,759, drilled the caprock at the Spindletop field near Beaumont, Tex., with relative ease. That venerable invention, within the first decade of the last century, could drill a scant fraction of the depth and speed of the modern rotary rock bit. The original Hughes bit drilled for hours; the modern bit now drills for days. Modern bits sometimes drill for thousands of feet instead of merely a few feet. Many advances have contributed to the impressive improvements in rotary rock bits.
In drilling boreholes in earthen formations using rolling-cone or rolling-cutter bits, rock bits having one, two, or three rolling cutters rotatably mounted thereon are employed. The bit is secured to the lower end of a drill string that is rotated from the surface or by downhole motors or turbines. The cutters mounted on the bit roll and slide upon the bottom of the borehole as the drill string is rotated, thereby engaging and disintegrating the formation material to be removed. The rolling-cutters are provided with cutting elements or teeth that are forced to penetrate and gouge the bottom of the borehole by weight from the drill string. The cuttings from the bottom and sides of the borehole are washed away and disposed by drilling fluid that is pumped down from the surface through the hollow, rotating drill string, and the nozzles as orifices on the drill bit. Eventually the cuttings are carried in suspension in the drilling fluid to the surface up the exterior of the drill string.
Rolling-cutter bits dominated petroleum drilling for the greater part of the 20th century. With improvements in synthetic diamond technology that occurred in the 1970s and 1980s, the fixed blade cutter bit or drag bit became popular again in the latter part of the 20th century. Modern fixed blade cutter bits are often referred to as “diamond” or “PDC” (polycrystalline diamond cutter bits) bits and are far removed from the original fixed bladecutter bits of the 19th and early 20th centuries. Diamond or PDC bits carry cutting elements comprising polycrystalline diamond compact layers or “tables” formed on and bonded to a supporting substrate, conventionally of cemented tungsten carbide, the cutting element being arranged in selected location on blades or other structures on the bit body with the diamond tables facing generally in the direction of bit rotation. Fixed blade cutter bits have the advantage of being much more aggressive during drilling and therefore drill much faster at equivalent weight-on-bit levels (WOB) than, for instance, a rolling-cutter bit. In addition, they have no moving parts, which make their design less complex and more robust. The drilling mechanics and dynamics of fixed blade cutter bits are different from those of rolling-cutter bits precisely because they are more aggressive in cutting and require more torque to rotate during drilling. During a drilling operation, fixed blade cutter bits are used in a manner similar to that for rolling-cutter bits, the fixed blade cutter bits also being rotated against a formation being drilled under applied weight-on-bit to remove formation material. The cutting elements on the fixed blade cutters are continuously engaged as they scrape material from the formation, while in a rolling-cutter bit the cutting elements on each rolling cutter indent the formation intermittently with little or no relative motion (scraping) between the cutting element and the formation. A rolling-cutter bit and a fixed blade cutter bit each have particular applications for which they are more suitable than the other. The much more aggressive fixed blade cutter bit is superior in drilling in a softer formation to a medium hard formation while the rolling-cutter bit excels in drilling hard formations, abrasive formations, or any combination thereof.
In the prior art, some earth-boring bits use a combination of one or more rolling cutters and one or more fixed blade cutters. Some of these combination-type drill bits are referred to as hybrid bits. Previous designs of hybrid bits, such as U.S. Pat. No. 4,343,371, to Baker, III, have used rolling-cutters to do most of the formation cutting, especially in the center of the hole or bit. Another type of hybrid bit is described in U.S. Pat. No. 4,444,281, to Schumacher, has equal numbers of fixed blade cutters and rolling-cutters in essentially symmetrical arrangements. In such bits, the rolling-cutters do most of the cutting of the formation while the fixed blade cutters act as scrapers to remove uncut formation indentations left by the rolling-cutters as well as cuttings left behind by the rolling-cutters. While such a hybrid bit improves the cutting efficiency of the hybrid bit over that of a rolling-cutter bit in softer formations, it has only a small or marginal effect on improving the overall performance in harder formations. When comparing a fixed blade cutter bit to a rolling-cutter bit, the high cutting aggressiveness of a fixed blade cutter bit frequently causes such bit to reach the torque capacity or limit of a conventional rotary table drilling systems or motors, even at a moderate level of weight-on-bit during drilling, particularly on larger diameter drill bits. The reduced cutting aggressiveness of a rolling-cutter bit, on the other hand, frequently causes the rolling-cutter bit to exceed the weight-on-bit limits of the drill string before reaching the full torque capacity of a conventional rotary table drive drilling system.
None of the prior art addresses the large difference in cutting aggressiveness between rolling-cutter bits and fixed blade cutter bits. Accordingly, an improved hybrid bit with adjustable cutting aggressiveness that falls between or midway between the cutting aggressiveness of a rolling-cutter bit and a fixed blade cutter bit would be desirable.